It was a mystery that threatened to overturn over a hundred years of scientific canon. How was it that both Pioneer spacecraft were traveling slower through space than predicted by physics? The answer, it turns out, had little to do with Newton or Einstein, and more to do with poorly thought out spacecraft design.

Both Pioneers 10 and 11 were launched in the early 1970s on a mission to explore the outer planets. But a few years after they were sent into space, NASA scientists started to notice that they were slowing down as they approached Saturn — 0.9 nanometers per second to be exact. Figuring that it had something to do with leftover propellant in the fuel lines, they, ahem, swept the issue under the carpet.

But not content to leave it at that, a group of scientists recently decided to get at the bottom of the infuriating "Pioneer Anomaly." The integrity of Einsteinian relativity was at stake, after all. Writing in SEN, Amanda Doyle walks us through how the mystery was solved:

In 2004, Slava Turyshev and his colleagues started to gather old data from the Pioneer craft to try to analyse the strange effect further. They wanted to find out as much as possible about the anomaly before suggesting a deep space mission to NASA.

The data they needed was Doppler data, which is the pattern of data that the craft send back to Earth, and telemetry data. The problem was that all of this data was stored on punch cards and magnetic tapes, making it difficult to use. The punch card data had to be digitised, and a vintage tape machine has to be salvaged so that the tapes could be read. The total amount of data collected was 43 gigabytes, which is a substantial amount for a 1970s mission.

They discovered that this strange effect was unique to the Pioneer spacecraft, as they rely on spinning for stability rather than having thrusters aligned along three axes like the Voyager spacecraft.

They eventually concluded that it was not strange physics causing the deceleration, but heat from the electrical subsystems and the decay of plutonium which was pushing back on the craft.

"The effect is something like when you're driving a car and the photons from your headlights are pushing you backward," said Turyshev, lead author of the paper published on 12 June in Physical Review Letters. "It is very subtle."

So yay to standard physics! And boo to those spacecraft designers of the early 1970s.